EPR study of the anisotropic interaction parameter of dichlorobis(η-cyclopentadienyl)vanadium in chloroform

The temperature dependent EPR line widths of a paramagnetic species, [dichlorobis(η-cyclopentadienyl)vanadium] or [V(η⁵-C₅H₅)₂Cl₂], abbreviated as Cp₂VCl₂, were measured in chloroform at X band. An analysis of the temperature dependent EPR line widths of Cp₂VCl₂ in chloroform in the motional narrowing region was carried out, using the four sets of the magnetic parameters available in the literature. It was found that only those from Petersen and Dahl [J. L. Petersen and L. F. Dahl, J. Am. Chem. Soc., 97 (1975), p. 6422.] could explain our line width data. Our analysis in chloroform is in agreement with our results in toluene and suggests that Cp₂VCl₂ exhibits anisotropic rotational diffusion with N = 2.7 ± 0.7 at an axis z′ = y, where N is the ratio R_∥R_⊥; R_∥ is the rotational diffusion constant along the molecular z axis; and R_⊥ is the rotational diffusion constant perpendicular to the molecular z axis. This value is consistent with the N value obtained from the Stokes-Einstein model (N = 3.3) and the allowed-values equation, AVE, with ρ_x = 3.02 ρ_y - 7.18. The AVE is not consistent with isotropic reorientation and is only consistent with axial symmetry if ρ_x = 1, ρ_y = 2.71. The anisotropic interaction parameter κ, which is a measure of the anisotropy of intermolecular interactions, was found to be 0.156. The κ value of 0.156 showed that Cp₂VCl₂ in chloroform is less slip than Cp₂VCl₂ in toluene (its κ is 0.044). Therefore, it seems that the κ value increases with increasing solute-solvent interaction. The stickiness factor (S) was calculated for Cp₂VCl₂ in chloroform and was found to be close to the slip limit (S ≈ 0.1), indicating that there is very slight tendency for the chloroform solvent molecules to stick to the surface of the diffusing Cp₂VCl₂ particles.